Set SubtitlesPosXCenter to false, then it should work as expected.By default (for compatibility reasons) the SubtitlesPosX specifies the coordinate of the *center* of the subtitle.

Thanks for the hint. I should divine... Finally I left SubtitlesPosXCenter=true changing SubtitlesPosX to half-of-screen-width instead because I want subtitles to be centered. Anyway, it wouldn't be bad to add this info to WME Help.